An interposer is used to connect terminal electrodes formed on a bottom surface of an integrated circuit to electrode patterns formed on a substrate in a process of mounting the integrated circuit on the circuit board.
In a related art process of mounting a semiconductor Integrated Circuit (IC) on a circuit board, leads extending from side surfaces of an IC package are inserted in through holes of lands of a circuit pattern formed on the circuit board, and are electrically connected to the lands by soldering. Recently, the number of input/output terminals of the IC is increasing as IC integration density increases. In addition, higher frequency performance is in demand as operating frequency of the IC becomes high. In view of these recent trends, mounting an IC on a circuit board with higher density, with shorter connection distances and with narrower pitches is now increasingly demanded.
In order to meet these demands, a technique has been proposed to mount input/output terminals on a circuit board using an interposer. The input/output terminals are arranged on a bottom surface of an IC package in a grid array pattern, which helps efficiently arrange the input/output terminals. An interposer is a high terminal density thin connector formed of an insulating material sheet having through holes formed at positions corresponding to the input/output terminals of the IC package arranged in a grid array pattern and having electric conductors (i.e., connectors) placed in the through holes for the conduction of the front and back surfaces of the insulation material. Terminal patterns are formed in a similar grid array pattern on the circuit board. Here, mounting of an IC package on a circuit board using an interposer will be described with reference to FIGS. 1A to 1C.
FIG. 1A illustrates an interposer 2 to which the present application is applied being disposed between a circuit board 3 and an IC package 1. FIG. 1B is a side view of FIG. 1A. In FIG. 1B, the interposer 2 is illustrated in cross section. Input/output terminals (i.e., electrodes) 4 are arranged in a grid array pattern on a back surface of the IC package 1. Terminal patterns (i.e., electrodes) 6 are formed at positions opposite to the input/output terminals 4 on the circuit board 3 on which the IC package 1 is to be mounted. Although not illustrated, other circuit patterns and electronic parts are provided on the circuit board 3 for the connection with the terminal patterns 6.
The interposer 2 is disposed between the IC package 1 and the circuit board 3 to connect the input/output terminals 4 formed on the back surface of the IC package 1 to the terminal patterns 6 formed on the circuit board 3. The interposer 2 is a high terminal density thin connector formed of an insulation material sheet (hereinafter, referred to as an interposer substrate) having through holes 9 formed at positions corresponding to the input/output terminals 4 of the IC package 1 arranged in a grid array pattern and having connectors 5 placed in the through holes 9. The connectors 5, which are of the same length, are electric conductors for the conduction of the front and back surfaces of the interposer substrate 8.
Usually, as illustrated in FIG. 1C, the interposer 2 is attached to an inner side of a socket 7, which in turn is mounted on the circuit board 3 by soldering or other means. The socket 7 facilitates mounting and dismounting of the IC package 1 on and from the circuit board 3.
In the thus-configured interposer 2, the structure of the connectors 5 as conductors for the conduction of the front and back surfaces of the interposer substrate 8 is important. The connectors 5, which will be pressed between the input/output terminals 4 formed on the back surface of the IC package 1 and the terminal patterns 6 formed on the circuit board 3, elastically compressed by the pressure from the upper and lower sides for the conduction between the input/output terminals 4 and the terminal patterns 6.
Japanese Laid-open Patent Publication No. 2004-47986 discloses elastic connectors (“electrically conductive contacts” in the Document) that are made of foam metal or a combination of foam metal and an elastomer. Foamed metal is produced by various processes from electrically conductive metals, such as copper, copper alloy, silver, silver alloy, gold, nickel and molybdenum. The foaming process includes: bubbling gas through molten metal, stirring a foaming agent through the molten metal, consolidating metal powder with a particulate foaming agent, pressure infiltrating the molten metal into wax or a foam polymer precursor, and performing a vapor deposition process for the deposition of metal onto the foam polymer precursor.